Magnetic sensors are used for a variety of applications to sense motion, position, and/or speed. Common applications for magnetic sensors in vehicles are engine controls and Anti-Lock Braking Systems (ABS). In both cases, magnetic sensors are placed in close proximity to a toothed disc or the like for measuring vehicle wheel speed. It is desirable to position the sensing magnet as closely as possible to the disc for maximum sensor performance.
U.S. Pat. No. 5,278,496 to Dickmeyer discloses a magnetic sensor design illustrated herein in prior art FIG. 3. This design includes an opening in the outer shell or casing which exposes an outer surface of the sensing magnet. In designs of this type, the magnet is mounted within the shell or casing by molding shell/casing material around sides of the magnet to hold it in place transversely and by overmolding a portion of the exposed magnet surface with shell/casing material to hold it in place axially. In this regard and with reference to prior art FIG. 3, edges of the magnet are typically provided with a radius and the overmolded shell/casing material grips this radius to press an inner surface of the magnet against the sensor pole piece and secure it within the sensor. The radius is generally provided by tumbling the magnets prior to assembling them--a practice which not only increases their production cost, but also results in some wastage.
A disadvantage of this design is that the shell/casing applies compressive/bending forces to the magnet which may cause it to crack and portions to dislodge from the sensor, degrading performance and possibly requiring replacement. These forces may be magnified by the vibration or shock of sensor distribution or of the vehicle environment, or by thermal expansion and contraction of the housing. All of these problems are exacerbated by the use of rare earth metal magnets which provide high performance and light weight but tend to be brittle and thus are easily cracked or chipped.
In other prior art designs, the sensing magnet is completely enclosed by .the sensor shell/casing material which is typically plastic. Plastic is the material of choice for the outer housing because of its durability, low cost, ease of forming and imperviousness to environmental effects. One such design is disclosed in U.S. Pat. No. 5,431,117 to Singbartl illustrated herein in prior art FIG. 4. This design provides some protection against magnet cracking, however, the plastic overcoating deleteriously affects performance of the sensor.
What is desired, therefore, is a magnetic sensor in which the sensing magnet is mounted in such a way so as to limit destructive compressive forces while retaining any magnet chips and obtaining a high level of performance.